The present invention relates generally to Josephson junctions in thin-film superconductors, and more particularly to Josephson junctions in high temperature oxide superconductors suitable for use in various electronic components.
The application of superconductor devices has been largely limited to experimentation because of the costly cryogenic requirements to operate below their transition temperatures, T.sub.C. However, a large number of superconductors with high transition temperatures are now commercially available, the highest currently being close to 125.degree. K in a Tl-based material. This enables use of more economical cryogenic refrigerants, such as nitrogen which liquifies at 77 K(-195.8 F), and a potential for a developing oxide superconducting electronic industry.
Common to all the high temperature superconductors is the copper oxide (CuO) planes, which produces a layered structure with large anisotropies in mechanical, electrical and thermal properties. They are brittle, opaque, hard and, at room temperature, poor electrical conductors. Another common property of these materials is their axis-dependent coherence length .xi., typically less than 50 .ANG..
YBa.sub.2 Cu.sub.3 O.sub.7, herein after referred to as SC-123, has a superconducting transition temperature T.sub.C =92.degree. K and is one of the easiest to prepare in single phase in either bulk or thin-film form. Others, such as Tl- and Bi- based oxide superconductors have higher transition temperatures but are more difficult to prepare in single-phase form due to the proximity of several closely related structures with different transition temperatures.
The fundamental superconductor device is the Josephson junction. It consists of two independent superconductors weakly coupled to each other by a coupling structure. There are four main types of coupling structures. These are the "classic" tunnel junction, layered structures, microbridges, and point contacts. To behave as a Josephson junction, the size of the coupling structure must be of the order of the coherence length .xi. of the superconductors. The coherence length .xi. of low temperature superconductors is several orders of magnitude larger than the crystal unit cell. This made it easy for fabrication of low temperature Josephson junction devices. In the new high temperature superconductors, however, the coherence length .xi. is of the order of the crystal lattice spacing or smaller. This means that the coupling structure must be of the order of a unit lattice spacing or less. The smallness of the coherence length has been at the root of the problem of making a reliable Josephson junction.
Conventional methods of making Josephson junctions, such as multi-layer deposition of a superconductor-normal-insulator-superconductor (SNIS), e.g. Nb-Al-A.sub.2 O.sub.3 -Nb, and microbridges, are not suitable for the new high temperature superconductors. The difficulty in making a microbridge junction is in the patterning of lateral dimensions of the order of 30 .ANG.. Larger microbridges, on the order 1 .mu.m (10,000 .ANG.) have been fabricated but the critical currents I.sub.C are so high that the bridges melt when they become normal, i.e. non-superconductive. In the case of SIS junctions, the coherence length .xi. along the crystallographic c-direction is in the order of 5 .ANG., which is about one-third of the unit cell size in that direction. Therefore, it is very difficult to grow a uniform thin layer of insulating material. There is also interdiffusion among different layers, and the possibility of electrical shorts between layers. Praseodymium (Pr) is the only rare earth element which can be substituted for yttrium(Y) in SC-123, and that is not a superconductor. Several research groups have tried to use PrBa.sub.2 Cu.sub.3 O.sub.7 to form a SIS structure because it has similar lattice parameters and thermal expansion coefficients as SC-123, but so far they have not demonstrated Josephson junction effects.